This invention relates to an organic-inorganic perovskite polymer compound having excellent photoemission characteristics and non-linear characteristics, and which can be applied to EL elements or time-space transformation elements.
More specifically, this invention relates to an organic-inorganic laminar polymer compound represented by the general formula A2MX4 where A is an organic ammonia molecule, M is a group IV element or transition metal, and X is a halogen, the compound having a structure stabilized by polymerizing the organic layer of the organic ammonia-inorganic halide laminar perovskite compound forming a super lattice structure (FIG. 1) wherein the organic ammonia molecule A layer and inorganic halide MX4 layer are laminated alternately.
The laminar perovskite compound represented by the general formula (RNH3)2MX4, as shown in FIG. 1, has a self-ordered quantum well structure wherein an inorganic semiconductor layer (halogenated metal (MX42xe2x88x92) and organic ammonium RNH3 dielectric layer are connected two-dimensionally by sharing the apex of a halogenated metal MX6 of octahedral structure. The bandgap of this organic dielectric layer is much larger than that of the inorganic semiconductor layer, so an electron is enclosed in the inorganic semiconductor layer. This electron is enclosed on the inorganic semiconductor layer two-dimensional surface (the structure is referred to as a xe2x80x9cquantum well structurexe2x80x9d). Due to this quantum well structure, this compound exhibits very intense photoemission characteristics and third-order non-linear optical characteristics.
The reason why (PbX)42xe2x88x92 is taken as the inorganic semiconductor layer (PbX4) is because, due to this low dimensional semiconductor structure, a stable exciton is formed having a flux energy as large as several hundred meV, so it has very interesting exciton characteristics such as a strong exciton absorption and photoemission even at room temperature. It has also been found that it has a large third-order non-linear light sensitivity factor of the order of 10xe2x88x925 esu, so optical material applications such as electroluminescence or optically excited laser emissions may be expected.
In particular, (CnH2n+1NH3)2PbI4 is the substance with the most remarkable exciton effect.
However, as these laminar perovskite compounds had a low stability with respect to light, heat and humidity, there were problems in their application. This instability is thought to be due to dissociation of halogen and fluctuation of amines in the organic layer induced by light.
It is therefore an object of this invention to increase the stability of the quantum well structure of laminar perovskite compounds having excellent optical characteristics and possible applications in light emitting elements.
Specifically, it is known that compounds having unsaturated bonds such diacetylene, due to their regular arrangement, are polymerized by applying external energy such as UV light or radiation. The organic amines in laminar perovskite compounds are oriented substantially perpendicular to the inorganic layer due to halogen ions in the inorganic layer, hydrogen bonds and Van der Waals forces. These have a regular arrangement due to the arrangement of metal. On the other hand, perovskite compounds have a high radiation resistance. Hence, by introducing unsaturated bonds such as double bonds or triple bonds into the organic layer and irradiating with a radiation, solid polymerization can occur in a regular structural state. In this way it is thought that, by polymerizing laminar perovskite compounds, fluctuations in the organic layer can be decreased.
In this invention, it was discovered that amines having unsaturated bonds can be introduced into the organic layer of organic-inorganic laminar perovskite compounds comprising a metal halide and an organic amine, and the organic layer is polymerized by applying external energy such as by irradiating with UV light or radiation. In this way, the quantum well structure is stabilized.
Specifically, in the following examples, it was evident that by introducing lead bromide, PbBr2, which might be expected to increase stability, into the inorganic layer, and an amine having an unsaturated bond such as a diacetylene bond or the like into the organic layer, and polymerizing these species, a highly stable organic-inorganic laminar perovskite compound can be obtained.
Further, by using this method, it is also possible to construct an organic-inorganic superlattice wherein the organic layer is not a simple obstacle, but is an active blocking layer having a conjugated structure into which functionality has been introduced.
This invention is also an organic-inorganic laminar perovskite polymer compound produced by cross-linking unsaturated bonds of an organic-inorganic laminar perovskite compound represented by the general formula (RNH3)2MX4.
In the formula, R is a hydrocarbon group having an unsaturated bond. This unsaturated bond may be either a double bond or triple bond, but a triple bond permits easier polymerization. Also, there is no particular limitation on the number of unsaturated bonds. There is no particular limitation on the number of carbon atoms in R, but it is preferable that the number of carbon atoms is suitable for polymerization, specifically of the order of 2-20. R may be straight chain or branched, but straight chain is preferable from the viewpoint of ease of polymerization. An example of R is the hydrocarbon group represented by CH3(CH2)nCxe2x89xa1Cxe2x80x94Cxe2x89xa1CCH2 (preferably, n=2-14). M is a Group IVa metal, Eu, Cd, Cu, Fe, Mn or Pd, preferably a Group IVa metal or Eu, more preferably a Group IVa metal, still more preferably Ge, Sn or Pb and most preferably Pb. X is a halogen atom, preferably Cl, Br or I, and most preferably Br. X may also be a mixture of halogens.
The method of cross-linking the organic layer, and particularly the method of cross-linking the unsaturated bonds contained in the organic layer, may be any method known in the art, but irradiation with ultraviolet light or radiation is convenient and preferred. The degree to which these unsaturated bonds should be cross-linked differs depending on the application and the molecular structure comprising the unsaturated bonds, and therefore is determined according to the case. It is not absolutely necessary to perform cross-linking until all the unsaturated bonds have been completely eliminated, and provided polymerization is continued until a target fluctuation has decreased to a predetermined degree, it may be considered that this purpose has been achieved. The polymerization conditions are a matter to be designed by the polymerization technician.
The organic-inorganic laminar perovskite polymer compound of this invention has excellent photoemission characteristics and non-linear characteristics, so its application is expected in EL elements or time-space transformation elements which demand these characteristics, and studies have already been performed on applications to EL elements or the like. It may be expected that these applications will be enhanced by this invention.
These polymers not only stabilize the organic layer, but also offer the possibility of forming a novel superlattice. For example, if diacetylene is polymerized it becomes polydiacetylene, and as polydiacetylene is a semiconductor, a quantum well structure different from an organic layer comprising an insulator can be manufactured. Further, as the organic layer also exhibits semiconductor characteristics, there is thought to be an interaction with the inorganic layer so that a novel superlattice structure is formed. This superlattice structure is an interesting structure which is expected to improve third- order non-linear optical characteristics. This invention will not only accelerate research on low dimensional exciton physics, but will also provide an important technique for developing new optically functional devices.